Characterization of transport of newly assembled, T cell-stimulatory MHC class II-peptide complexes from MHC class II compartments to the cell surface.

In human B cells MHC class II molecules acquire antigenic peptides in lysosome-related compartments called the MHC class II compartments (MIIC). How assembled complexes, capable of activating T cells, then reach the cell surface has not been fully resolved. We have used selective ablation of early and recycling endosomes to determine whether newly peptide-loaded class II molecules require functional recycling endosomes to exit to the cell surface. Cellular compartments accessed by transferrin-horseradish peroxidase conjugates were functionally inactivated by cross-linking with diaminobenzidine and hydrogen peroxide. Cells with ablated endosomal compartments were unable to recycle transferrin to the cell surface and could not deliver exogenous Ag for processing and presentation to T cells. In contrast, cells that had taken up Ag and assembled intracellular class II-peptide complexes before endosome ablation were still able to deliver class II-peptide complexes to the cell surface and stimulate T cell proliferation. This delivery was abolished in the presence of brefeldin A. These data show that the parts of the endocytic apparatus necessary for Ag delivery to the MIIC are not required for functional class II-peptide complexes to reach the cell surface. Moreover, the brefeldin A sensitivity of this final step in the class II molecule biosynthetic pathway suggests a vesicular intermediate for transport between the MIIC and the plasma membrane.